1. Field of the Invention
This invention relates to environmental test chamber heating and cooling systems, and more specifically, to an improved method of cooling environmental test chambers using an lower capacity, smaller footprint cascade refrigeration unit in combination with a thermal storage unit.
2. Description of the Related Art
Environmental test chambers subject components within them to a variety of physically challenging test conditions. These test conditions can include acceleration tests, sand or water tests, and temperature tests. The temperature tests can consist of not extremes of heat and cold, but also tests of large temperature change in very short periods of time. A typical environmental test chamber system for imposing large temperature changes in very short periods of time comprises either a single or twin section insulated environmental test chamber, and coupled to the environmental test chamber, a large capacity refrigeration system. A large capacity environmental test chamber system is capable of imposing a temperature change from +150.degree. C. to -65.degree. C. in the span of five minutes, and reducing the temperature to -73.degree. C. Additionally, slower tests utilizing temperature ramp rates of five, ten, or 20.degree. C. per minute are also within this field, still have large system capacity requirements.
A more complete explanation of environmental test methods and standards is detailed in: the Electronics Industries Association's, (EIA) JEDEC JESD22 group of specifications; Military Specifications Mil-Std 202, Mil-Std 750, Mil-Std 810, and Mil-Std 883; and the IEC pub 68 IEC Standards, all of which are incorporated herein by reference.
The physical plant requirements to produce these temperature changes, whether the very fast temperature ramp rate or the slower ramp rates, are very substantial. A large tonnage refrigeration system is required, and the physical size of such a large capacity refrigeration system is correspondingly large. A large tonnage refrigeration system also has substantial energy requirements while it is in operation. An additional problem with conventional environmental test chamber systems is that the temperature transient, from the hot extreme to the cold extreme, for cyclic testing may be quite large. In order to subject the item under test to the desired temperature transition, the item under test in an environmental test chamber system must either: (1) be physically moved from a first pre-heated hot chamber into a second pre-cooled cold chamber, a physical transition that requires two separate and insulated chambers which results in a system with a double size facilities footprint; or (2) for a single chamber environmental test chamber system, this refrigeration system must be larger yet to enable the sudden heat transfer of the item under test's heat load.
Therefore, a need existed for an improved environmental test chamber refrigeration system that has the requisite temperature transition capabilities utilizing a smaller capacity cascade refrigeration system for single chamber environmental test chambers. Another need existed for an improved environmental test chamber refrigeration system that has the requisite temperature transition capabilities utilizing a smaller capacity cascade refrigeration system for dual chamber environmental test chambers. A further need existed for an improved environmental test chamber refrigeration system having only one insulated environmental chamber thereby eliminating the physical movement of an item under transition temperature testing and also providing a reduced facilities footprint. Yet a further need existed for an improved environmental test chamber refrigeration system having an improvement in energy usage efficiency.